1. Field of the Invention
The present invention relates to fabrication processes involving glass and the like and, more particularly, to fabrication processes involving multiple-layered materials systems, such as glass systems used in the fabrication of electronic apparatus, as, for example, gas panel display devices and the like.
2. Description of the Prior Art
One of the difficulties encountered in fabrication processes requiring the assembly of glass parts resides in the fact that the temperature needed to carry out one step of the fabrication process may be detrimental to previously assembled parts. For example, the temperature required for a fusing or welding step may be such as to cause the softening and deformation of previously fabricated and assembled glass parts. Typically, such a problem might be encountered in the manufacture of glass articles.
Exemplary of the above problems are those involved in the manufacture of glass articles, such as cathode ray tubes, wherein it may be necessary as one of the final steps to seal the tube by fusing the glass faceplate to the glass body of the tube. In such a sealing process, the temperature required to fuse the glass faceplate to the body of the tube may be such as to cause damage to prefabricated parts within the glass tube.
One obvious approach employed heretofor to solve the particular problem of damage caused by the relatively high temperatures involved in fusing and sealing glass articles, is to endeavor to find low-melting sealants. Such an approach is employed, for example, by J. Francel et al in U.S. Pat. No. 3,127,278, entitled, "Low-Melting Glass Sealant and Article Made Therefrom." Typical of other low-melting glass for use as a frit (a powder), soldering material, sealant, and the like, are those described by K. Ikeda et al. in U.S. Pat. No. 3,420,683 and 3,425,817. Likewise, F. Veres describes, in U.S. Pat. No. 3,645,839, a low temperature glass sealant containing aluminum titanate. However, as will be explained more fully hereinafter, the low-melting sealant approach is not always successful, where sealing temperature creates the problem.
It should be noted in this regard, that the problem of damage to glass parts due to relatively high temperatures is not necessarily limited to sealing operations required in glass tubes and the like, but may be prevalent fabrication any of a variety of fbrication operations involving glass systems. For example, it is evident that glasses employed for encapsulation must, in general, be relatively low-melting glasses, for otherwise damages to previously formed parts might occur during their flow out operation. Likewise, when one glass layer is deposited upon another, the temperature required for flowing out the former on the latter must be, in general, below the deformation or reflow temperature of the latter, where deformation or reflow must be considered in the context of a time-temperature dependent phenomenon.
It should be understood here that in the fabrication of devices involving glass systems, that a certain amount of softening or reflow in previously formed glass parts therein undergoing some form of temperature processing does not necessarily pose a problem. The degree of softening or reflow which is tolerable in a particular glass part undergoing some form of temperature processing depends upon the nature and function of the part, and how such softening or reflow affects other parts associated therewith. For purposes of description, "reflow" as used herein is intended to mean any "softening", reflow, "excessive reflow", "melting", etc., that is undesirable or intolerable by way of being deleterious to device integrity such as to affect device operation, performance, reliability, life, etc. Typically, reflow that causes even minimal deformation or distortion of previously formed glass parts is, in a great number of present day applications, intolerable, particularly as pertains to applications involving electronics. It should be noted that glasses typically will reflow and deform at any temperature above the glass transition temperature. In this regard, it should also be noted that reflow in glass may occur at any temperature during which the time-temperature cycle involved provides sufficient heat to cause a discernably deleterious change in the glass part being processed.
In fabricating electronic apparatus having composite structures of glass layers, or glass, metal and crystalline layers, for example, it is clear that it is necessary, in general, that the hierarchy of successive different layers achieve sufficiently low viscosities so as to enable flow out to take place at succeedingly lower temperatures, such that as the fabrication steps progress, lower and lower temperatures are required therefor. In particular, as each successive layer of the composite structure is fabricated, it must soften and flow out at a lower temperature than that of any of the previously applied layers. The necessity of this declining flow out temperature hierarchy most often limits design choice. Also limiting design choice is the fact that the magnitude of the coefficient of thermal expansion of each of the successive layers must be compatibly close. However, it is obvious that the above conditions must generally prevail in order to maintain practical structural integrity.
The problem of damage to prefabricated glass parts due to the heat required to carry out fabrication of composite structures is compounded by the fact that design considerations may, at times, require the use of a glass in such structures that deforms at temperatures at or below the temperatures required for subsequent steps in the process. Typically, such subsequent processing steps may involve a sealing operation. As previously mentioned, although various efforts have been made to obtain relatively low temperature sealants, where an unusually low temperature glass is necessary at some point in the fabrication of particular apparatus, it may not be possible to obtain a sealant exhibiting a sufficiently low sealing temperature such that deformation or reflow of the low temperature glass is avoided.
Likewise, design considerations may at times require a structure to be made having a considerable number of layers of glass. Since each successively fabricated layer of glass must normally be produced at a temperature lower than the deformation or reflow temperatures of the glasses employed in fabricating the underlying glass layers thereof, it is clear that a final sealing step, for example, may be required by the glass hierarchy to be made at a temperature lower than the sealing temperature of available sealants.
Accordingly, it is clear that the application temperature of available low-temperature sealants may at times not be sufficiently low so as to be compatible with the rest of the structure involved with the sealing operation. For example, one approach to the fabrication of a.c. gas panel display devices requires that a dielectric glass with a relatively low flow out temperature be fabricated in layer form upon both plates of a pair of conventional relatively low softening point glass plates, having an array of conductors deposited thereon. Typically, commercially available plate glass from LOF (Libby-Owens-Ford) or ASG (American Saint Gobain) are employed for the substrate plate. Thereafter, a thin layer of metal oxide, such as MgO, is deposited upon the glass dielectric layer. As one of the final steps in the fabrication process, the pair of glass plates are sealed together to form a gas tight chamber. However, difficulty has been encountered during the sealing process. Available low-melting glass sealants require a temperature which is, by comparison, high enough to cause some reflow of the previously flowed on glass dielectric layer. The reflow of the glass dielectric layer causes crazing of the overlying thin layer of metal oxide, (such as MgO) and in addition reacts with it. This crazing is intolerable for device performance.
Not only does the device performance require that the above mentioned crazing be avoided but, in addition, to retain structural integrity it is required that, in accordance with practical design constraints, the coefficient of thermal expansion of the various parts of the glass system involved therein vary only slightly from one part to another. However, to devise a glass system wherein the coefficients of thermal expansion of the various parts are compatible with one another and, yet, wherein each successive fabrication step is performed at a temperature sufficiently low so as to not reflow or deform previously formed parts, is quite difficult with the normally available selection of materials.
In regard to the latter, one would normally generally expect in fabricating glass systems, that mixtures of glasses from the normally available selection of materials, in the range of compositions tried, typically would show crystalline or non-crystalline phase separation. In addition, in such mixtures it is to be expected that the properties of the resulting glass are unpredictable. Accordingly, in addition to showing discontinuities at phase boundaries, it can be expected that relatively large and unpredictable variations in the coefficients of thermal expansion and viscosity may exist.
Thus, whether the fabrication problems confronted in glass systems are incident to the fabrication of a.c. gas panel display devices, or any of the variety of electronic apparatus and the like which use glass systems, the problems, to a large degree, are the same.